Investigators have tallied up the number of records stolen in an attack on the U.S. Office of Personnel Management (OPM), and it's bigger than anyone thought.

The agency has concluded "with high confidence" that hackers got away with sensitive information including Social Security numbers on 21.5 million people -- almost everyone who underwent a background security investigation for a government job through OPM since 2000.

The majority of records, some 19.7 million, were for background investigation applicants while an additional 1.8 million were from nonapplicants -- friends and family of applicants who would also be investigated as part of the process.